The distinguishing characteristic of a heat shrinkable film is the ability upon exposure to some level of heat to shrink or, if restrained, to create shrink tension within the film. These responses are initiated when the film is passed through a hot air or steam shrink tunnel. The resulting shrinkage of the film results in an aesthetically pleasing transparent wrapping which conforms to the contour of a product while providing the usual functions required of packaging materials such as protection of the product from loss of components, theft, or damage due to handling and shipment. Among the many items wrapped in polyolefin shrink films are cans, bottles, plastic containers, toys, games, sporting goods, stationery, greeting cards, hardware and household products, office supplies and forms, foods, phonograph records, and industrial parts.
Heat shrinkable films are required to possess a variety of controllable attributes. For example, in additional to a high shrink response, shrink films must also possess good optical and mechanical properties.
There are two main categories of heat shrink films, hot-blown shrink film and oriented shrink film. Hot-blown shrink film is usually made by a hot-blown bubble film process; conversely, oriented shrink film is made by biaxial and uniaxial orientation processes including double bubble, simultaneous longitudinal and transverse orientation which is sometimes referred to as LISIM® technology, tape bubble, trapped bubble or tenter framing. The use of linear motors to directly propel tenter clip to effect simultaneous longitudinal and transverse orientation is disclosed in U.S. Pat. No. 4,853,602. Both amorphous and semi-crystalline polymers can be made into oriented shrink films using elaborate biaxial orientation processes. For amorphous polymers, the orientation is performed at a temperature immediately above the glass transition temperature of the polymer. For semi-crystalline polymers, the orientation is performed at a temperature below the peak melting point of the polymer. General uniaxial orientation techniques are disclosed in U.S. Pat. No. 6,436,496. Uniaxially oriented films for use as dry cell battery labels are disclosed in U.S. Pat. No. 5,747,192.
Blown and oriented films are an important class of industrial products useful in the wrapping or packaging of various articles of commerce. Oriented isotactic polypropylene film is known to be useful for its high barrier properties, clarity and stiffness. Polypropylene polymers used for oriented films are typically isotactic homopolymers, or blends of isotactic polypropylene polymers with differing melt flow rates (MFR) or tacticity indices, or blends of isotactic polypropylene with atatic polypropylene, syndiotactic polypropylene, random polypropylene copolymers having minor amounts of ethylene or higher alpha-olefins, and ethylene copolymers. See, for example, U.S. Pat. Nos. 4,950,720 and 5,691,043. As disclosed in U.S. Pat. No. 4,921,749, other components such as petroleum resins may be added to modify properties such as heat-sealing performance, gas permeability and stiffness. Such oriented polypropylene, including biaxially oriented polypropylene (BOPP) films, have many desirable properties for use in the packaging and labeling of products. U.S. Pat. No. 6,436,496 discloses heat shrinkable films which may have a core layer with up to 55 wt. % of a polyolefin material with the balance being an ethylene-propylene copolymer.
The packaging and labeling of a wide variety of products in films, both transparent films as well as non-transparent films has for some time included packaging articles in heat shrinkable film. Heat shrinkable film can be used to obtain a tight package by placing an item into a bag or sleeve fabricated from a heat shrink film, then closing or sealing the bag by heat sealing, solvent sealing, or sealing with a hot melt adhesive, and thereafter exposing the bag to sufficient heat to cause shrinking of the bag and intimate contact between the bag and item. The heat that induces shrinkage can be provided by conventional heat sources, such as heated air, infrared radiation, hot water, hot oil combustion flames, or the like. Heat shrink wrapping of food items helps preserve freshness, is attractive, hygienic, and allows closer inspection of the quality of the packaged food item. Heat shrink wrapping of industrial and retail goods, which is alternatively referred to as industrial and retail bundling, preserves product cleanliness and also is a convenient means of bundling and segregating for transporting purposes. Similarly, heat shrinkable film may be used for labeling by wrapping the film around the article to be labeled and applying heat. The film will shrink causing the label to conform to the article thereby creating a tight-fitting and attractive label for the article.
In such packaging operations, it is frequently desirable to maintain the air or water within the shrink tunnel or water bath, at the lowest effective temperature to accomplish the desired level of film shrinkage, in order to avoid subjecting the packaged or labeled product to a temperature so high that there is an undesirable effect on the product. Such is particularly the case in the packaging of food products, such as fresh meat products which can be scorched by higher temperatures, even upon exposure to the relatively high temperature for only a brief period of time, for example, 30 seconds. Likewise, in the packaging of cheese products, low shrink temperatures are desirable in order to prevent discoloration of the cheese within the package. From an energy efficiency standpoint, it is also desirable to shrink the films at lower temperatures.
Furthermore, in the packaging of relatively rigid products which are not distorted by forces produced by a shrinking film, it is generally desirable to provide a heat shrinkable packaging film with as high a shrink response as possible, in order to provide the “tightest” possible packaging over the product. In general, a tighter package or label provides an appearance more pleasing to the purchaser of the article packaged or labeled. This is especially true with respect to packaging of highly contoured rigid products such as industrial mechanical components and bone-in meat products.
The shrink response of shrink film is obtained by initially stretching fabricated film to an extent several times its original dimensions to orient the film. This stretching may occur in a single direction or in multiple directions, depending on the process employed. For example, in hot-blown process, the film is stretched in all directions, in biaxially oriented films, the stretching occurs in both the machine and transverse directions and in the machine direction only for uniaxially oriented films. The stretching is usually accomplished while the fabricated film is sufficiently soft or molten, although cold drawn shrink films are also known in the art. After the fabricated film is stretched and while it is still in a stretched condition, the stretching or orientation is set by quickly cooling the film. Subsequent application of heat causes the oriented film to relax and, depending on the actual shrink temperature, the oriented film can return substantially back to its original unstretched dimensions, i.e., to shrink relative to its stretched dimension. In the prior art, most roll-fed MD (machine direction) heat shrinkable polyolefin films exhibit shrinkage of about 25% at 135° C. Typically, these films exhibit haze values of greater than 5% with gloss of less than 90%. Haze values referred to in this application are determined in accordance with ASTM D1003 testing method and gloss values are determined in accordance with ASTM D2457. The films often have poor storage stability exhibiting considerable shrinkage when stored at temperatures of 60° C. or higher for 14 days or more. For example, as exemplified in U.S. Pat. No. 5,747,192, conventional films typically retain less than 95% of their shrinkage after high temperature storage. Conventional films may also have poor seam integrity when used in conjunction with hot melt adhesives in label applications.
In order to provide a film with a higher shrinkage, it is generally necessary to orient the film to a higher degree. Conventional polyolefin based films having MD shrinkage greater than 25% at 135° C. generally exhibit problems in maintaining high optical quality upon undergoing shrinkage, especially substantial shrinkage. That is, such high shrink films undergo substantial “whitening” upon shrinkage, as well as a substantial reduction in gloss. Also, when high uniaxial shrink film is produced, typically a high MD stretch ratio must to be used in combination with lower MD stretch temperatures. This results in TD (transverse direction) stretch lanes or bands across the web. These stretch bands cause bagginess in the film, which limits the use of the film for further converting. Delamination of the film is also a potential problem during the shrinkage of such high shrink films. It is desirable to provide a film that maintains the highest level of transparency and gloss obtainable. The loss of optical properties of the prior art high shrink films is detrimental in many packaging and labeling applications.
Certain applications, e.g., labeling, covering, or packaging of materials such as boxes, plates, vessels, bottles, tubes, cylindrical material, e.g., pipes and rods, etc. are particularly suitable applications for heat shrinkable films. However, in certain situations it is desirable to affect shrinkage along a single axis without substantial shrinkage in the cross-direction. For example, in the process of labeling bottles by shrinking a tube of heat shrinkable material, if the film shrinks along its length, the label may not be positioned correctly but rather placed above the desired position upon shrinkage. Moreover, printing and other conversion processes of such label surfaces require heat stability in substantially one direction to meet machinability requirements. Uniaxially shrinkable materials can also be used in preparing tightly wrapped containers by lap heat sealing uniaxially shrinkable film resulting in shrinkage of the film.
In order to obtain uniaxially shrinkable materials it is possible to employ uniaxially oriented materials, i.e., materials which are oriented in only one direction. However, uniaxially oriented film can lack the requisite strength and toughness necessary for use in such applications. In as much as biaxially oriented films exhibit desirable strength and tear resistance in both directions of orientation, it is desirable to obtain a uniaxially heat shrinkable film which is biaxially oriented and thus substantially stable in the cross-direction.
For more detailed information on heat shrinkable films, reference may be found in U.S. Pat. Nos. 4,194,039; 3,808,304; 4,188,350; 4,194,039; 4,377,616; 4,390,385; 4,448,792; 4,582,752; and 4,963,418. The manufacture of polypropylene heat shrinkable films using modifiers in a polypropylene based shrink layer to make films suitable for packaging, labeling, and other applications is disclosed in U.S. Pat. Nos. 5,691,043; 5,888,640; 6,303,233; and 6,322,883.